1. Field of the Invention
The present invention relates to a device for limiting the working voltage for mechanical switches in telephony.
2. Discussion of the Related Art
As is known, a switch, technically termed a "hook", is used in telephone sets to connect the telephone set itself to the telephone line when the handset is lifted. This hook carries, when activated, a line current which in some cases reaches a value of 120 milliamperes.
Traditionally, in electromechanical sets the hook comprised an actual mechanical switch, as better explained hereinafter. In recent times the mechanical switch has been completely or partially replaced with electronic switches, such as bipolar transistors or MOSFET transistors, due both to their lower costs and to their higher reliability, which is the result of the elimination of problems related to the oxidation of the contacts of the switch due to sparking, and other minor problems.
However, in order to drive these electronic switches it is still necessary to use a mechanical or magnetic switch to detect the lifting of the handset from its normal on-hook state.
In this case one obtains the simplification of the mechanical switch, of the simple-contact type, as well as the reduction of the currents which flow through the switch itself, which can be estimated in the range of a few microamperes.
The voltage that in any case remains applied across these switches, when the switch is open, is equal to the voltage present on the telephone line. In some cases, this line voltage can be of several hundred volts, for example in the case of a call as signal for the ringer or during tests performed on telephone lines, and is limited only by the value set by the circuit which protects against atmospheric discharges, i.e. lightning.
In the presence of such high voltage values it is currently necessary to follow precautions, since the humidity of the air can cause the false closure of the switch due to a significant reduction in insulation and thus produce a false closure detection which thus causes an associated actual closure of the electronic line switch.
An object of the present invention is to eliminate or substantially reduce the problems described above by providing a device for limiting the working voltage for mechanical switches in telephony, which device limits the voltage across the mechanical switches present in electronic sets in order to eliminate false closure signals.
Another object of the present invention is to provide a limiting device that allows the use of low voltage/current switches.
Yet another object of the present invention is to provide a limiting device which is highly reliable, relatively easy to manufacture and has a competitive cost.